Re:stop the prescriptivism (Score 1)

There is a reasonable point to be made for the idea that consistancy is valuable. However, I'm not sure I buy your implication that a non-brain-damaged twelve-year-old has an imperfect understanding of semantics or "underlying structure," which I take to mean syntax. If these kids didn't have good syntax, they wouldn't reliably produce things exactly in line with adult speech, and you would expect them to misparse adult speech with some noticeable effect. No such effect, to my knowledge, has been found.

In short: you can't teach language per se to kids. All you can do is try to teach them some of your language conventions, which may be worthwhile if you think your language is in some way superior to theirs, or if you want to enforce consistancy.